A significant retardation of radionuclides leaking from an underground repository can be expected if large parts of the rock body act as a sink for the radionuclides. To calculate the retardation, it is necessary to know the sorption properties and the diffusivities in the rock matrix for the radionuclides in the rock. The diffusivity will determine to what extent the rock matrix may be penetrated. Sorption experiments have been performed to determine the diffusion and sorption properties of cesium and strontium in crushed granite particles with one granite from Finnsjoen outside Forsmark on the east coast of Sweden, and one granite from the Stripa mine in central Sweden. Granite samples have been crushed and screened, and six different particle size fractions from 0.10 to 0.12 mm and 4 to 5 mm of each rock have been used in the experiments. The initial concentrations of inactive cesium and strontium were 10 to 15 ppm. A “synthetic” groundwater was used. The adsorption isotherm was found to be linear for strontium but nonlinear for cesium. One conclusion from this is that a prediction of cesium migration velocity from one single distribution coefficient is inappropriate. The experimental data indicate that the amount of sorption is dependent not only on the mass of granite particles but also to some extent on the size of the particles. A distinction has been made between sorption on external surfaces and inner surfaces. The amount of external surface adsorption was found to vary from 15 to 40% of the total adsorption capacity for the particle size fraction of 0.10 to 0.12 mm to a few percent or less for the largest particles used. Except for the largest particles, the experimentally determined diffusivities were found to lie in the interval expected from literature data on electric conductivities. The diffusivities were found to increase with increasing particle size. This could be explained by a higher diffusion rate in grain boundaries than in a homogeneous material. Nearly all of the smallest particles consist of only one mineral each.